Pulse counter frequency modulation detection



Defedor, M Fi/fer, Pulse Counkr r Phase June 11, 1968 N. SCRIBNER3,388,333

PULSE COUNTER FREQUENCY MODULATION DETECTION Original Filed April 12,1963 2 Sheets-Sheet 1 Demodulafor' Shif'fl' l I no Law Pass Fi/ferINYENTOR. Neal Scrub/7e! wu q 514 June 11, 1968 N. SCRIBNER 3,388,333

PULSE COUNTER FREQUENCY MODULATION DETECTION Original Filed April 12,1963 '2 Sheets-Sheet 2 Fig.4.

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United States Patent ice 3,388,333 PULSE COUNTER FREQUENCY MODULATIONDETECTION Neal Scribner, Independence, Mo., assignor to Wilcox ElectricCompany, Inc, Kansas City, Mo., :1 corporation of Kansas Originalapplication Apr. 12, 1963, Ser. No. 272,734, now Patent No. 3,351,864,dated Nov. 7, 1967. Divided and this application Aug. 22, 1967, Ser. No.662,504

6 Claims. (Cl. 329-404) ABSTRACT 0F THE DISCLOSURE A radio navigationreceiver is adapted to receive a space modulated carrier component and afrequency modulated subcarrier component of a navigation signal and hasan AM detector which passes the space modulation and the subcarrier.Subsequent filter circuitry separates the space modulation and thesubcarrier, the latter being fed to a pulse counter demodulator fordetection of the modulating signal. The pulse counter demodulatorutilizes bistate switching means responsive to a predetermined, lowamplitude level of each half cycle of the subcarrier for producing atrain of generally rectangular pulses which are fed to succeedingcircuitry where the modulating signal is recovered. Such modulatingsignal and the detected space modulation are then fed to separate inputsof a phase angle meter where a phase comparison of the two detectedsignals is effected.

This application is a divisional application of my copending applicationSer. No. 272,794, filed Apr. 12, 1963, now Us. Letters Patent No.3,351,864, granted November 7, 1967, and entitled Pulse CounterFrequency Modulation Detection.

This invention relates generally to frequency modulation detectionsystems and, more specifically, to an improved radio navigation receiverincluding an improved pulse-counter type demodulator.

A commonly employed VHF navigational system for aircraft utilizes thephase difference between a pair of detected radio signals as a means ofindicating to the pilot or navigator of an aircraft the position of suchaircraft relative to a ground reference point from which such signalsare emanating. For example, in the basic VHF omnidirectional rangesystem a high frequency carrier signal is radiated from a rotatingtransmitting antenna which has a 360 angular sweep. The transmittedcarrier contains a subcarrier or side band which is frequency modulatedat a frequency equal to the sweep frequency of the antenna. Means isemployed to maintain the amplitude of the subcarrier at a constant levelregardless of antenna position.

The receiver in an aircraft within the range of these signals is awareof an amplitude modulation on the carrier due to the swinging of thedirectional antenna. This is commonly referred to as space modulationand as aforesaid, is of the same frequency as the frequency modulationsimpressed upon the subcarrier. Therefore, after detection of the spacemodulation and of the modulating signal impressed upon the subcarrier, aphase comparison is made by the receiver circuitry to ascertain thephase difference between the two detected signals. Commonly, the spaceand frequnecy modulations are placed on their respective signals suchthat receivers located due north of the transmitting station will detectthe modulations in phase, while receivers located other than due northof the station will receive the modulations out-of-phase by a phaseangle characteristic of that particular azimuth.

3,388,333 Patented June 11, 1968 From the foregoing it may beappreciated that any phase errors introduced into the detected carrieror subcarrier signals will necessarily result in an erroneous indicationof the position of the aircraft relative to the 5 transmitting station.The probability of such error is es- 5 frequency modulation detectorwill be out-of-phase with the frequency modulating signal by an amountcorresponding to the severity of the amplitude modulation detection.

It is, therefore, an object of this invention to provide aphase-comparing radio navigation receiver which is' unresponsive to anyamplitude variations that may be present in the frequency modulatednavigation signal received thereby.

It is another object of this invention to provide a phase-comparingradio navigation receiver employing a pulse-counter type frequencymodulation demodulator.

It is another object of this invention to provide an improved method ofdemodulating a frequency modulated, periodic, electrical signal, whichmethod is uneffected by amplitude modulation of the signal.

It is another object of this invention to provide a pulse-counterdemodulator of minimum size and weight employing no vacuum tubecomponents or bulky filter networks, thus especially adapting thedemodulator for use in aircraft receivers.

It is still another object of this invention to provide a pulse-counterdemodulator employing a hybrid tunnel diode-transistor circuitresponsive to the incoming sig nal, electrical means responsive to theoutput of the hybrid circuit for producing a series of pulses of equalamplitude and width regardless of the period of the incoming signal, andoutput means responsive to the series of pulses for providing an outputsignal having an amplitude level proportional to the rate of recurrenceof said pulses.

It is yet another object of this invention to provide a pulse-counterdemodulator employing a hybrid tunnel diode-transistor switching circuitresponsive to a frequency modulated, incoming signal for producing afirst series of electrical pulses, a clamped differeutiator networkresponsive to said first series of pulses for producing a second seriesof electrical pulses, a capacitor discharge circuit responsive to saidsecond series of pulses for increasing the duration of each pulse ofsaid second series to a predetermined fixed width, a transistor switchhaving its input coupled with the capacitor discharge circuit andoperable thereby to produce at its output a train of electrical pulsesof equal amplitude and width, and an integrator network coupled with theoutput of the transistor switch for integrating said pulse train toprovide an output signal having an amplitude level proportional to thefrequency of the incoming signal.

Other objects will become apparent as the detailed description proceeds.

In the drawings:

FIGURE 1 is a block diagram of the improved radio navigation receiver;

FIG. 2 is a schematic circuit diagram of the pulsecounter demodulator ofthe present invention;

FIG. 3 is a graph illustrating the operation of the hybrid tunneldiode-transistor switching circuit;

FIG. 4 is a graph showing the output characteristics of thepulse-counter demodulator; and

FIGS. SA-SE are graphs showing the voltage wave forms at various pointsin the circuitry of FIG. 2 to be described fully hereinafter.

Referring to FIG. 1, it may be seen that a receiving antenna is coupledwith an amplitude modulation detector 12 having its Output coupled witharms 14 and 16 of the phase-comparing circuitry. More specifically,using the above-described VHF omnidirectional range navigation system asan example, the space modulated carrier component and the frequencymodulated subcarrier component of the signal emanating from thetransmitting antenna are received by antenna 10 and conducted to thedetector 12. Detector 12, which may employ as preceding stages an RFamplifier, a converter and an IF amplifier as in conventionalsuperheterodyne receiver systems, then blocks the high frequency carrierand permits the subcarrier or side band component and the spacemodulation on the carrier component to pass to the succeeding circuitry.

A filter 18 which may be of the band-pass or high-pass type is employedin arm 14 to eliminate the space modulation and allow the frequencymodulated subcarrier to pass substantially unattenuated. It may beappreciated that the space modulation will be of quite low frequency,frequently on the order of 30 c.p.s. and thus the separation of the twosignals by conventional filter networks may readily be accomplished.

The subcarrier is then fed to a pulse-counter demodulator 20 where themodulating signal is detected. The modulating signal is then conductedto a phase angle meter 22 to be described more fully hereinafter.

Arm 16 contains a low-phase filter 24 which permits the passage of thelow frequency space modulation therethrough and which suppresses thehigher frequency subcarrier. An amplifier 26 may then be employed toboost the amplitude level of the detected space modulation prior tointroducing the same to phase angle meter 22.

The pulse counter demodulator such as illustrated at 20 for extractingthe intelligence from the frequency modulated subcarrier is asubstantial improvement over demodulators employed heretofore innavigation receivers. Pulse-counter demodulators are unresponsive to anyamplitude changes or modulations appearing on the sub carrier and thusare not subject to the phase errors discussed above. In radio navigationsystems, amplitude modulation of the subcarrier may occur due tounforeseen space modulation effects, identification tones trans mittedby the ground station, identification broadcasts by voice, other voicetransmissions such as weather reports, etc. and interference. Within thereceiver itself, nonlinearity and intermodulation in the detector 12 mayalso induce amplitude modulation of the subcarrier.

The phase angle meter 22 comprises a phase shifter or phase shiftnetwork 28, an amplifier 30, a phase comparator 32, and an indicatordevice 34. Phase comparator 32 is responsive to a predetermined phasedifference between signals appearing at its inputs 36 and 38. When thisphase difference actually exists at terminals 36 and 38, indicator 34 soindicates by the centering of an indicator needle or the like.

Phase shifter 28 is an adjustable device capable of shifting the phaseof an electrical signal during the passage of such signal from its input40 to its output 42. The phase shifter is adjusted by the pilot duringoperation of the receiver until the needle or other indicating means ofthe device 34 centers. By appropriate calibrations of the phase shifteradjustment the pilot is made aware of the azimuth of the aircraftrelative to the ground transmitting station. Alternatively, of course,the phase shifter may be set at the desired azimuth and the course ofthe plane altered by the pilot until the needle of indicator 34 centers.This procedure and the internal circuitry of phase angle meter 22 shownin FIG. 1, is conventional in the radio navigation art and need not befurther dealt with in detail in this specification.

Referring to FIGS. 2-5E, an improved pulse-counter demodulator is shown.In FIG. 2 circuitry of the demodulator is schematically illustrated andcomprises an input terminal 44, a hybrid switching circuit having atunnel diode 46 and an NPN transistor 48 as components thereof, aclamped differentiator circuit comprising a capacitor 50, a resistor 52,and a diode 54, a capacitor discharge circuit comprising a capacitor 56and a resistor 58, an NPN switching transistor 60, an integrator circuitgenerally designated 62, and an output terminal 64. Electrical power issupplied from a direct current source (not shown) having its positiveterminal connected to bus 66 and its negative terminal grounded. Itshould be understood that ground serves as the other electrical side ofboth input terminal 44 and output terminal 64.

More specifically, tunnel diode 46 comprises an anode 46a which isconnected to junction point 68, and a cathode 46b which is connected toground. Transistor 48 has an emitter 48a connected to ground, a base 48bconnected to junction point 68, and a collector 48c. A resistor 70couples junction point 68 with input terminal 44, resistor '76 beingemployed as an impedance matching device so that the circuitry connectedto input terminal 44 will present a relatively high impedance source tothe input of the hydrid switching circuit.

The collector 480 is connected to the positive bus 66 at junction point76. This connection is effected through series connected resistors 72and 74, interconnected at junction point 75, which set the operatingcharacteristics of the input circuit of transistor 48. Junction points68 and 76 are interconnected by resistor 78. The function of resistor 78will be described when the operation of the circuitry is discussedhereinafter.

Capacitor 56 of the differentiator network is connected between junctionpoint and junction point 80. Resistor 52 of the differentiator networkis connected between junction point 80 and the positive bus. Diode 54has its anode 54a connected to junction point 80 and its cathode 54bconnected to ground.

The capacitor 56 and resistor 58 of the capacitor dis charge circuit areserially connected at junction point 82. The other end of resistor 58 isconnected to the high voltage bus, while the other lead from capacitor56 is connected to ground. A series circuit of capacitor 56 and resistor58 is thus formed across the terminals of the power source, such seriescircuit being coupled with the preceding circuitry by a clipping diodehaving its anode 84a connected to junction point 82, and its cathode 84bconnected to junction point 80.

Switching transistors 60 has an emitter 60a connected to ground, a base6011 connected to junction point 82, and a collector 660. An adjustableresistor 86 serially connected with a resistor 88 of fixed value,couples collector 63c with the positive bus 66.

Integrator network 62 comprises an RC filter consisting of resistors 90and 92 and capacitors 94 and 96. Resistors 90 and 92 are interconnectedin series at junction point 98. The other end of resistor 90 isconnected with the variable tap or slider of variable resistor 86, whilethe other end of resistor 92 is connected to output terminal 64.Capacitor 94 is connected across junction point 98 and ground, whilecapacitor 96 is connected between output terminal 64 and ground.

The operation of the circuitry of FIG. 2 will now be described.Referring to FIG. 5A, the graph illustrates the sinusoidal voltage waveform of an incoming electrical signal appearing at input terminal 44.(In the radio navigation system discussed above, the signal illustratedin FIG. 5A is the frequency modulated subcarrier emanating from theoutput of filter 18.) It may be noted that the signal shown in FIG. 5Ais of constant frequency and period, thus illustrating an unmodulatedcarrier signal.

When such carrier is frequency modulated, of course, the frequency willvary in accordance with the modulating signal and the period of thecarrier will increase and decrease correspondingly.

Referring to FIG. 3, curve 100 is the characteristic curve of tunneldiode 46, curve 102 illustrates the input characteristic of transistor48, and curve 104 is a net input characteristic of the tunneldiode-transistor combination obtained by graphical addition of curves100 and 102. Point 106 on curve 104 illustrates the quiescent operatingpoint of the tunnel diode-transistor combination. At such point, acurrent corresponding to the ordinate or current axis coordinate ofpoint 106 flows through resistor 78 and tunnel diode 46. Under suchcondition, transistor 48 is in the nonconductive state as may beappreciated from FIG. 3. The presence of the input voltage wave form asshown in FIG. 5A, however, at junction point 68, increases the voltagethereof with respect to ground, and the current level rises to point 108placing the tunnel diode and the transistor in heavy conduction inaccordance with tunnel diode switching characteristics well known in theart.

This switching point corresponds to point 110 on the input voltage waveform. Subsequently, when the input wave reaches a potential ofsubstantially the same magnitude but of opposite polarity, the currentthrough the tunnel diode-transistor combination is reduced to a levelcorresponding to point 112 on the composite curve 104. Since the actionof the tunnel diode will then move the operating point from 112 to thestable state at the same current level represented by point 113, theemitter-collector circuit of transistor 48 ceases to conduct. Thiscorresponds to point 114 on the input voltage wave form.

It may be appreciated from the foregoing that the ohmic value ofresistor 78 is selected such that switching will occur at a pair oftime-spaced amplitudes of the input signal which are of substantiallyequal, relatively low magnitude but of opposite polarity. This assuresthat the tunnel diode-transistor combination will operate in the samemanner, regardless of amplitude fluctuations in the input signal. Thecircuitry is thus rendered unresponsive to amplitude modulations on theinput signal, it being requisite only that the amplitude of the inputsignal reach a value sufficient to switch the hybrid switching circuit.In practice, these amplitudes represented at 110 and 114 will beextremely low relative to the amplitude peaks of the input signal due toamplification thereof prior to introduction of the signal to thecircuitry of FIG. 2. The output voltage wave form appearing at collector48c of transistor 48 is shown in FIG. 5B. This wave form shows thepotential at the collector with respect to the emitter 48a or groundduring the switching sequence above described.

It should be understood from FIGS. 5A to SE that points on the waveforms shown in the figures in vertical alignment are coincident in time.Furthermore, points lying above the time axes of the figures indicatevoltages of positive polarity, while points lying below the time axisrepresent voltages of negative polarity. Therefore, the negativeexcursion 116 of the wave form shown in FIG. 5B coincides in time withpoint 110 on the wave form of FIG. 5A.

From the foregoing it may be appreciated that when the input signalcauses transistor 48 to be switched into the conductive state, thevoltage at the collector 48c drops very rapidly along excursion 116 tozero along line 118. This initiates the production of a pulse 119.Subsequently, at point 114 on the input wave form, transistor 48 isswitched back into the nonconductive state and the collector voltagerises along positive excursion 120 to the level indicated at 122. Level122, of course, is essentially the same as the potential differenceexisting between positive bus 66 and ground. It should be noted that thewidth of the voltage pulses at collector 48c will vary with the periodof the input wave, but remain of constant am-- plitude. Furthermore, thewidth of the voltage pulses will be equal to the spacing therebetweenfor all input signal frequencies.

The voltage pulses from transistor 48 are differentiated by thedifferentiator network comprising capacitor 50 and resistor 52. Elements50 and 52 comprise a conventional diiferentiator network and produce anoutput voltage at junction point having a wave form as shown in FIG. 5C.The action of diode 54 clamps the diiferentiator output therebysubstantially reducing the positive voltage spikes. This clamping actionis vividly shown by the wave form in FIG. 5C wherein the voltage pulsecorresponding to the negative excursion 116 is shown at 124, and theclamped pulse corresponding to positive excursion 120 is shown at 126.The minute pulse 126 is present due to the nonlinear characteristics ofthe diode 54. The small quiescent voltage level 128 exists due to theresistance of the diode in the forward conduction direction.

The series of pulses 124 from the ditfentiator network are fed to thecapacitor discharge circuit comprising capacitor 56 and resistor 58through diode 84. The function of diode 84 is to clip the small spike orpulse 126 and isolate the discharge path of capacitor 56. FIG. 5D showsthe voltage wave form appearing at junction point 82 with respect toground and clearly illustrates that the capacitor discharge circuitserves to increase the width of pulses 124 to a predetermined duration.Pulses 130 commence coincident in time with corresponding pulses 124 butterminate at times illustrated by points 132. Each pulse 130 commenceswith a sharp negative excursion 134 and decays along a negative ramp 136corresponding to the discharge of capacitor 56.

Since the train of negative pulses 130 appears across the emitter-basejunction of transistor 60, the transistor is rapidly switched into thenonconductive state by the leading edge of each pulse. Similarly, theemitter-collector circuit of transistor 60 returns to the conductivestate as the trailing edge of each pulse 130 passes into the region ofpositive potential. This is illustrated in FIG. 5B wherein the train ofoutput voltage pulses appearing at the collector 60c is shown. Thevertical edge 138 of each pulse 140 is coincident in time with thecorresponding excursion 134, and the tapered trailing edge 142 occursapproximately coincident with the corresponding point 132; between edges138 and 142 the pulse is of constant amplitude as shown at 144.

It may be appreciated that each pulse 140 will be of the same amplitudeand width, the only variation in the train of pulses 140 being in thespacing between adjacent pulses.

The adjustable resistor 86 serves as a gain control and establishes theamplitude of pulses 140. The integrator circuit 62 is a network ofconventional design which produces at output terminal 64 a signal havingan amplitude level proportional to the rate of recurrence of pulses 140.

Referring to FIG. 4, the output characteristic of the invention isshown. Line 146 vividly illustrates the linear response characteristicsof the output signal appearing at terminal 64. It will be appreciatedthat as the frequency of the incoming signal varies, due to modulationthereof, the voltage level of the output signal appearing at terminal 64will vary proportionally. This occurs until a maximum input signalfrequency is reached where the width of each pulse 140 is equal to theperiod of the input signal.

Having above described the apparatus of the present invention, themethod aspect thereof may now be readily appreciated. The method taughtby the invention of demodulating a frequency modulated, electricalsignal comprises the following steps.

First, a first series of electrical pulses is generated responsive tothe signal, each of the pulses being triggered by and commencingcoincident in time with a predetermined characteristic of the wave formof a correspond ing periodic recurrence of the signal and terminatingduring said corresponding periodic recurrence. The triggering of thepulses is illustrated by FIGS. A and 5B wherein the points 116 on thevoltage wave form of the incoming signal correspond to the negativeexcursions 116 of pulses 119. Termination of each pulse 119 is shown at120 which corresponds to point 114 on the incoming signal wave form.

Secondly, the first series of pulses is electrically differentiated toobtain a train of electrical pulse pairs, each pulse of the pulse paircommencing coincident in time, respectively, with the rise and fall of acorresponding pulse of the first series of pulses. This is illustratedin FIGS. 5B and 5C wherein pulses 124 and 126 correspond, respectively,with excursions 116 and 120 of pulses 119.

Thirdly, a second series of electrical pulses is produced responsive toevery other pulse of the train of pulse pairs, the pulses of said secondseries being of equal durations, each of which is triggered by andcommences coincident in time with the corresponding pulse of the trainof pulse pairs. This step is illustrated in FIGS. 5C and 5D where it maybe seen that the minute pulses 126 are eliminated when pulses 130 areproduced.

Fourthly, a third series of electrical pulses is generated responsive tothe second series of pulses, the pulses of the third series being ofequal amplitudes and widths, each of which is triggered by and commencescoincident in time with the corresponding pulse of the second series ofpulses. This is accomplished by applying the second series of pulses toa bistate, electrically responsive switching device having a source ofconstant, direct voltage coupled with the load-switching terminalsthereof. (Such a device is shown in FIG. 2 as transistor 69, the emitter60a and collector 60c constituting the load-switching terminalsthereof.) In this manner the third series of pulses is obtained at theload-switching terminals. This step is shown in FIGS. 5D and 5E whereinpulses 13f} represent the second series of pulses, and pulses 140represent the third series of pulses.

Fifthly, the third series of pulses is integrated to obtain an outputsignal 'whose amplitude level is proportional to the frequency of theincoming signal.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. Apparatus for receiving a signal having an amplitude modulatedcarrier component and a frequency modulated, periodic subcarriercomponent comprising:

detector means responsive to said signal and adapted to have said signalapplied thereto for demodulating said carrier component and passing saidsubcarrier component;

first frequency selective means coupled with said detector means forselectively passing said subcarrier component;

second frequency selective means coupled with said detector means forselectively passing said demodulated carrier component;

a pulse counter demodulator coupled with said first frequency selectivemeans and responsive to said subcarrier component for detecting thelatter; and

output means coupled with said pulse counter demodulator and said secondfrequency selective means and responsive to said detected subcarriercomponent and said demodulated carrier component for comparing thephases thereof and indicating the phase relationship therebetween,

said demodulator comprising means responsive to said subcarriercomponent for producing a train of pulses of substantially equalamplitude and width, each of said pulses commencing during acorresponding periodic recurreneeof said subcarrier component, and meansresponsive to said pulse train for producing an output signal having anamplitude level proportional to the recurrence rate of said pulses,whereby said subcarrier component is detected.

2. Apparatus for receiving a signal having an amplitude modulatedcarrier component and a frequency modulated, periodic subcarriercomponent comprising:

detector means responsive to said signal and adapted to have said signalapplied thereto for demodulating said carrier component and passing saidsubcarrier component;

first frequency selective means coupled with said detector means forselectively passing said subcarrier component;

second frequency selective means coupled with said detector means forselectively passing said demodulated carrier component;

a pulse counter demodulator coupled with said first frequency selectivemeans and responsive to said subcarrier component for detecting thelatter; and

output means coupled with said pulse counter demodulator and said secondfrequency selective means and responsive to said detected subcarriercomponent and said demodulated carrier component for comparing thephases thereof and indicating the phase relationship therebetween,

said demodulator comprising first means responsive to a pair oftime-spaced amplitude levels of said subcarrier component for producinga series of electrical pulses successively commencing and terminating insubstantial time coincidence with one of said levels and the other ofsaid levels respectively, whereby each of said pulses occurs during acorresponding eriodic recurrence of said subcarrier component, andsecond means responsive to said series of pulses for deriving therefroman output signal having an amplitude level which varies in accordancewith changes in the frequency of said subcarrier component, whereby thelatter is detected.

3. The invention of claim 2,

said second means including circuit means coupled with said first meansfor producing a train of output pulses of substantially equal amplitudeand width, each output pulse occurring in response to a correspondingpulse of said series of pulses, and output 'means coupled with saidcircuit means and responsive to said train of output pulses forproviding said output signal with said amplitude level thereof beingproportional to the rate of recurrence of said output pulses.

4. Apparatus for receiving a signal having an amplitude modulatedcarrier component and a frequency modulated, periodic subcarriercomponent comprising:

detector means responsive to said signal and adapted to have said signalapplied thereto for demodulating said carrier component and passing saidsubcarrier component;

first frequency selective means coupled with said detector means forselectively passing said subcarrier component;

second frequency selective means coupled with said detector means forselectively passing said demodulated carrier component;

a pulse counter demodulator coupled with said first frequency selectivemeans and responsive to said subcarrier component for detecting thelatter; and

output means coupled with said pulse counter demodulator and said secondfrequency selective means and responsive to said detected subcarriercomponent and said demodulated carrier component for comparing thephases thereof and indicating the phase relationship therebetween,

said demodulator comprising electrically responsive,

bistate switching means responsive to a pair of timespaced amplitudelevels of said subcarrier component for producing a series of electricalpulses successively commencing and terminating in substantial timecoincidence with one of said levels and the other of said levelsrespectively, whereby each of said pulses occurs during a correspondingperiodic recurrence of said subcarrier component, circuit means coupledwith said switching means for pr ducing a train of output pulses ofsubstantially equal amplitude and width, each output pulse occurring inresponse to a corresponding pulse of said series of pulses, and outputmeans coupled with said circuit means and responsive to said train ofoutput pulses for providing an output signal having an amplitude levelproportional to the rate of recurrence of said output pulses, wherebysaid subcarrier component is detected.

5. Apparatus for receiving a signal having an ampli tude modulatedcarrier component and a frequency modulated, periodic subcarriercomponent comprising:

detector means responsive to said signal and adapted to 'have saidsignal applied thereto for demodulating said carrier component andpassing said subcarrier component;

first frequency selective means coupled with said detector means forselectively passing said subcarrier component;

second frequency selective means coupled with said detector means forselectively passing said demodulated carrier component;

a pulse counter demodulator coupled with said first frequency selectivemeans and responsive to said subcarrier component for detecting thelatter; and

output means coupled with said pulse counter demodulator and said secondfrequency selective means and responsive to said detected subcarriercomponent and said demodulated carrier component for comparing thephases thereof and indicating the phase relationship therebetween,

said demodulator comprising electrically responsive,

'bistate switching means responsive to a pair of timespaced amplitudelevels of said subcarrier component for producing a first series ofelectrical pulses successively commencing and terminating in substantialtime coincidence with one of said levels and the other of said levelsrespectively, whereby each of said pulses occurs during a correspondingperiodic recurrence of said subcarrier component, circuit means coupledwith said switching means and responsive to said first series of pulsesfor electrically differentiating the latter, whereby to provide a secondseries of electrical pulses, electrical means coupled with said circuitmeans and responsive to every other pulse of said second series ofpulses for producing a train of output pulses of substantially equalamplitude and width, each of which commences substantially coincident intime with a corresponding pulse of said second series of pulses, andoutput means coupled with said electrical means and responsive to saidtrain of output pulses for providing an output signal having anamplitude level proportional to the rate of recurrence of said outputpulses, whereby said subcarrier component is detected.

6. The invention of claim 5,

said amplitude levels of said subcarrier component being positive andnegative respectively and low rela tive to the peak amplitudes of thesubcarrier component.

References Cited UNITED STATES PATENTS 2,988,695 6/1961 Leavitt 324823,128,437 4/1964 Loughlin 329'126 X 3,172,053 3/1965 Ronzheimer 329-126X 3,351,864 11/1967 Scribner 329109 X ALFRED L. BRODY, Primary Examiner.

